While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, the commonly used light exposure lithography is approaching the essential limit of resolution determined by the light source wavelength.
As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp has been widely used. One means believed effective for further reducing the feature size is to reduce the wavelength of exposure light. For the mass production process of 64 M-bit DRAM, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 1 G or more requiring a finer patterning technology (processing feature size 0.13 μm or less), a shorter wavelength light source is required. In particular, lithography using ArF excimer laser light (193 nm) is on the verge of mass scale production.
In search for a light source of a shorter wavelength, lithography using a F2 laser (157 nm), known as F2 lithography, is under investigations. Even the use of F2 lithography encounters difficulty in complying with the demand for micropatterning to a feature size below 100 nm, especially below 65 nm. Candidates for the technology include high-energy electron beam direct writing, low-energy electron beam direct writing, high-energy ion beam, high-energy electron reduction projection lithography (EPL), low-energy electron beam proximity lithography (LEEPL), extreme ultraviolet lithography (EUV), and the like.
With the progress toward a finer feature size, resist pattern collapse becomes serious. Since a pattern having an aspect ratio of at least 3 undergoes a margin drop due to resist pattern collapse, efforts are made to reduce the thickness of resist film. Since a thinner resist film has lower resistance to dry etching, this propensity accelerates a reduction of the margin for the thickness of a resist film for processing a substrate.
Known methods for processing an extremely thin resist film include a resist process using an organic antireflective coating as an undercoat below the resist film, and multilayer resist processes such as bi- and tri-layer resist processes.
The resist undercoats used in these processes include organic films having an optical constant effective for antireflection, silicon-containing organic and inorganic films having etching resistance, and organic films having a structure for achieving a high carbon density. Particularly when organic films are prepared, it is a common practice to add acid crosslinkers and thermal acid generators for the purposes of forming a robust film, preventing intermixing with an overlying resist layer, and avoiding footing of a resist pattern.
In the multilayer resist process, acid crosslinkers and thermal acid generators added to resist undercoat materials play an important role. To promote crosslinking reaction by heating, thermal acid generators of generating a strong acid are necessary. Then thermal acid generators capable of generating perfluoroalkylsulfonic acid are advantageously used. Undesirably low molecular weight sulfonic acids such as trifluoromethanesulfonic acid and nonafluorobutanesulfonic acid are so volatile that they can volatilize off during heat crosslinking reaction. On the other hand, perfluorooctanesulfonic acid and analogues having a long-chain alkyl group (collectively referred to as PFOS) are less volatile, but suffer from toxic and environmental problems including their stability (or non-degradability) due to C—F bonds, and biological concentration and accumulation due to hydrophobic and lipophilic natures. The US EPA adopted Significant New Use Rule, listing 13 PFOS-related chemical substances and further listing 75 chemical substances although their use in the photoresist field is excluded. Because of the toxic and environmental problems, it would be desirable to develop PFOS-free materials.
Facing the PFOS-related problems, manufacturers made efforts to develop partially fluorinated alkyl sulfonic acids having a reduced degree of fluorine substitution. For instance, JP-A 2004-531749 describes the development of α,α-difluoroalkylsulfonic acid salts from α,α-difluoroalkene and a sulfur compound and discloses a resist composition comprising a photoacid generator which generates such sulfonic acid upon irradiation, specifically di(4-tert-butylphenyl)iodonium 1,1-difluoro-1-sulfonate-2-(1-naphthyl)ethylene. JP-A 2004-002252 describes the development of α,α,β,β-tetrafluoroalkylsulfonic acid salts from α,α,β,β-tetrafluoro-α-iodoalkane and a sulfur compound and discloses a photoacid generator capable of generating such a sulfonic acid and a resist composition comprising the same. JP-A 2002-214774 discloses a number of photoacid generators capable of generating partially fluorinated alkyl sulfonic acids and resist compositions comprising the same, although it lacks synthesis examples of specific compounds. However, these substances still have many problems regarding the availability of starting intermediates and the difficulty of their preparation. These substances are used only as photoacid generators while their use in undercoat material is referred to nowhere.